iface-helper: fix non-reentrant call to platform for failed IPv6 DAD

Platform invokes change events while reading netlink events. However, platform code is not re-entrant and calling into platform again is not allowed (aside operations that do not process the netlink socket, like lookup of the platform cache).

That basically means, we have to always process events in an idle handler. That is not a too strong limitation, because we anyway don't know the call context in which the platform event is emitted and we should avoid unguarded recursive calls into platform.